Control Flow: Topic introduction

In this part of the course, we will cover the following concepts:

• Control flow structures and the practice of writing of modular code
• Conditional statements and blocks
• For loops, while loops, and list comprehensions

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Getting from point A to point B

Control flow

Control flow structures

• The directions that allow us to control the flow of a program are called control flow structures in programming languages
• Control flow structures are very similar across languages in their concept… (but not syntax!)

Basic control flow structures in Python

• We will discuss the following control flow structures:

  • Conditionals (if-else statements)
  • Loops (for and while structures)
  • Functions (def structure)

Control flow structure capabilities

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Conditional statements

Conditional statements (cont’d)

if 2 + 2 == 4:
    print('Good, we are sane!')
else:
    print('We are living in 1984!')

Good, we are sane!

Recap: Logical operators

• The most common way to create Booleans (i.e. true or false values) is by using logical operators
  
  

  Operator	Example
  Greater than	x > y
  Less than	x < y
  Equal to	x == y
  Not equal to	x != y
  Greater than or equal to	x >= y
  Less than or equal to	x <= y

Recap: Combining Booleans

• We can use and and or to check for a combination of conditions

# Check if 2 conditions are true 
# for the expression to return `True`!
x = 8
print(x > 5 and x < 10)

True

• We can use add to add as many statements as we want, but when Python checks the conditions, it will only return True if all of the conditions are met

# Every single condition much be true for this expression to return `True`!
print(x > 5 and x > 1 and abs(x) == 7 and x < 10)

False

• Since abs(x) is equal to 8 not 7, the entire expression returns False

Condition types

• Conditions vary in many ways, but they absolutely need to result in a true or false output
• Conditional expressions (e.g. 2 + 2 == 4) can also be assigned to variables
• Long conditional statements are often more readable that way within if-else blocks

condition = 2 + 2 == 4
print(condition)

True

another_condition = "this string" == "that string"
print(another_condition)

False

yet_another_condition = 5 + 10 > 10 + 5
print(yet_another_condition)

False

Condition types (cont’d)

• Conditional statements can be compound (i.e. consist of multiple conditions that result in a single true or false output)

compound_condition1 = (5 + 10 > 10 + 5) and ("this string" == "this string")
print(compound_condition1)

False

compound_condition2 = (5 + 10 >= 10 + 5) and ("this string" == "this string")
print(compound_condition2)

True

# Here, it's helpful to look at each condition individually.
# If one of them is true, then the whole statement is true.
compound_condition3 = (compound_condition1 and compound_condition2) or (100/2 > 100 % 2)
print(compound_condition3)

True

Putting it all together

• Here is an if-else statement with a compound condition written directly within it

if (5 + 10 > 10 + 5) and ("this string" == "this string"):
    print('Compound condition 1 is true, do something!')
else:
    print('Compound condition 1 is false, do something else!')

Compound condition 1 is false, do something else!

• Now here is the same if-else statement with the compound condition saved to a variable beforehand

compound_condition1 = (5 + 10 > 10 + 5) and ("this string" == "this string")

if compound_condition1:
    print('Compound condition 1 is true, do something!')
else:
    print('Compound condition 1 is false, do something else!')

Compound condition 1 is false, do something else!

• Both ways are correct

Special cases of conditional blocks

• Conditional blocks of code don’t have to be in the if-else form
• Sometimes you want to take action if the condition is true, but don’t need to do anything if it is false
• In that case, just a single if block will do the trick!

if compound_condition2:
    print("Ok, I guess I have to do something after all!")

Ok, I guess I have to do something after all!

if yet_another_condition:
    print("This means the `compound_condition3` is true, otherwise you will get nothing!")

• In the case of the first example, the action is triggered, because compound_condition2 is true
• In the second example, the print statement is not triggered because yet_another_condition is false, and we gave our program no alternative action to perform

Special cases of conditional blocks (cont’d)

• If we want to check multiple conditions, we can use elif
• As soon as the program finds one condition that is satisfied, it will stop running down the elif tree
• Suppose you wrote a program to help you make a decision about purchasing a car based on its price

price = 37000

if price > 40000:
    print("That's too expensive!") 
elif price > 34000:
    print('A little pricey but maybe worth it...')
elif price > 26000: 
    print("This seems like a fair price for the quality")
elif price > 22000:
    print("What a good deal! I'll get it")
else:
    print("Hmmm this is pretty cheap, maybe there's a problem with it.")

A little pricey but maybe worth it...

Special cases of conditional blocks (cont’d)

• Sometimes we have a complex set of decisions to make based on the outcome of the other set of decisions
• In that case, nested conditionals are the way to go

if condition:
    if another_condition:
        print("You got to a nested statement!")
    else:
        print("You still got to the nested statement!")
else:
    print("No luck printing a nested statement!")

You still got to the nested statement!

Actions within conditional blocks

• The last element of conditional blocks are actions
• Virtually anything can go into actions associated with if and elif blocks

price = 37000
account_balance = 45000

if price > 38000:
    action = "Leave the dealership immediately, this is a rip off!"
    account_balance = account_balance - price
elif price > 22000 and price <= 38000 :
    action = "Take the car and go celebrate, you can afford it!"
    account_balance = account_balance - price
else:
    action = "Leave the dealership immediately, this is a scam!"
    account_balance = account_balance - price
    
print(action)

Take the car and go celebrate, you can afford it!

print("Current account balance:", account_balance)

Current account balance: 8000

Actions within conditional blocks (cont’d)

• Let’s change price and print results

price = 41000
account_balance = 45000

if price > 38000:
    action = "Leave the dealership immediately, this is a rip off!"
    account_balance = account_balance - price
elif price > 22000 and price <= 38000 :
    action = "Take the car and go celebrate, you can afford it!"
    account_balance = account_balance - price
else:
    action = "Leave the dealership immediately, this is a scam!"
    account_balance = account_balance - price
    
print(action)

Leave the dealership immediately, this is a rip off!

print("Current account balance:", account_balance)

Current account balance: 4000

Conditionals and actions based on them

• Any action can be housed within a conditional block

• Actions we might take include:

  • Performing mathematical operations
  • Assigning variables new values
  • Building other conditional blocks within them (a.k.a. nested conditional blocks)
  • Adding loops
  • Calling functions or even other programs

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